None of the standard software used for mining enables adding additional possible transactions manually AFAIK. It theoretically could be done though.

The only thing you need is to be capable of actually mining a block, which costs about 13k USD I've heard.

yeah, don't broadcast the transaction publicly. Only give it to the miner you're colluding with.

However, in some blockchains it is absolutely feasible to become a majority miner for a short period of time by simply renting cloud resources.

If the network is further temporarily reduced due to internet connectivity flapping/segmentation, it might be cheap.

1) Situate yourself in a country with poor connectivity. 2) Trip a breaker to zap most of the connectivity 3) Spin up a gazillion cloud instances 4) Compete more-or-less only with national miners.

If you've already mined the next block yourself, you can publish the transaction directly in the block itself instead of loose out to the network like a pleb. Then you'll be effectively guaranteed to get the transaction fee yourself.

0. You run a large network with substantial hash power.

1. You submit a transaction that is broadcast only to your own mempool and don't relay it outside of your network.

2. You attempt to include this output in some/most/all attempts.

3. You finally solve a block with this transaction and distribute it to the rest of the network.

If this were happening, one way to surveil for it would be to keep track of every submitted transaction that a few nodes ever see. Is there a mining pool that is consistently mining huge fee transactions that were never present in your mempool?